1. Field of Invention
The present invention relates to a phase-locked loop (PLL), and more particularly to a PLL capable of rapid lock-in.
2. Related Art
A PLL is an automatic control circuit system capable of tracing the frequency and phase of an input signal, which mainly traces and locks the phase and frequency of an output signal and an input signal such that the phase and frequency of the output signal and the input signal tend to be the same, and at this moment, they are referred to as being locked.
PLLs may be generally classified into analog PLLs and digital PLLs. Taking the system architecture of a common digital PLL as an example, referring to FIG. 1, it is a system block diagram of a conventional digital PLL. As shown in FIG. 1, the digital PLL 10 includes a phase-frequency detector (PFD) 100, a phase difference quantizer 110, and a digital controlled oscillator (DCO) 120, and a divider 130 is further added if different multiple frequencies are required. Since the basic concept of the conventional analog PLL is similar to that of the digital PLL and is well known by those skilled in the art, it will not be described herein.
Referring to FIG. 2, it is a corresponding diagram of frequency to period of the reference signal, wherein the horizontal axis represents time, and the vertical axis represents frequency. During rising time of a square wave, a conventional analog PLL or a digital PLL will reach a lock-in state generally after generating a plurality of oscillation periods 201. For operating frequencies above MHz, a time period of several microseconds (μs) or even less is required to achieve a lock-in state, and also being closely related to the reference signal frequency. Even with greater bandwidth, the conventional PLL locks-in after oscillating scores or hundreds of periods. Therefore, even for some practical applications with low reference signal frequency such as video systems and display systems, the stability and operating speed of the PLL still do not meet their needs, which is a problem yet to be solved.